The present invention relates to a method for determining the inclination of a tower, in particular of a wind turbine, in relation to the gravitational field, by evaluating the output signal from an acceleration sensor configured to pick up static acceleration in the direction of a sensor measurement axis.
The problem of shifting foundations arises specifically in offshore wind turbine constructions. The distinctive vibrating ability of the elongate wind turbines further intensifies the problem of the shifting of the wind turbines. To determine undesirable shifts of this type which, in the case of wind turbines, could have all kinds of undesirable effects, such as uneven stresses on bearings, there is therefore a need for generic methods which determine the inclination.
The measurement is difficult, since the wind turbine can often be vibrating while the measurement is being carried out, which is associated with accelerations which influence the measuring signal of an acceleration sensor with an amplitude which significantly exceeds that of the measuring signal produced by the mounting.
EP 2 133 563 A1 discloses a method of this type in which the acceleration, speed, position and inclination of the nacelle are determined as a result of a signal evaluation which comprises a plurality of steps and is based on measured values from an acceleration sensor in the nacelle of the wind turbine.
Another method, used in practice, for determining the inclination of wind turbines is based on the bubbles of a spirit level being read out via a video camera. This method is unsatisfactory in terms of measurement in every respect. A further problem is that the inclination to be measured can be so small that tilts in relation to the tower vertical axis of the acceleration sensor used for the measurement could falsify the measurement result beyond recognition.
It is therefore the object of the present invention to provide a method for determining the inclination of a tower of the type mentioned at the outset, which is simple to use and can be implemented on any wind turbines, for example by measurement personnel using a mobile measuring device, for example within the framework of routine tests.
This object is achieved according to the invention by a method for determining the inclination of a tower of the type mentioned at the outset, in which method the acceleration sensor is attached to a component, preferably to a main frame, which can be rotated about the longitudinal axis of the tower within an azimuth angular range of at least 180°, preferably of 360°, such that the sensor measurement axis is oriented substantially parallel to the plane of rotation of the component, the output signals being measured and recorded successively in the case of different azimuth angles, in which the component is rotated between measurements, the inclination being determined by evaluation of the series of measurements obtained thus. According to the invention, an acceleration sensor for example can be used, which is already present for other reasons, usually for measuring the vibration of the wind turbine. Compared to known methods for measuring inclination using acceleration sensors, recording the output signal in the case of different azimuth angles of the nacelle has the advantage that a large number of data points functionally connected to the required measured value can be obtained, which allow a good statistical averaging of the measured values. This is very advantageous on account of the initially mentioned vibrations under which the measurement object will usually stand. Acceleration sensors which are suitable within the context of the invention include all known acceleration sensors which can also measure constant accelerations in order to be able to detect the gravitational acceleration and the direction thereof. For example, devices with a movable test mass can be used in the simplest case. However, systems based on flexible quartz rods or magnetically stabilised masses are also possible. Finally, microelectromechanical systems (MIMS) are measured for the inclination. In any case, it is important that the devices only output the acceleration along one measurement axis. In particular, the acceleration sensor in the context of the invention can be fitted around the tower axis of a rotatable machine house. The series of measurements is ideally carried out for different azimuth values in the region of 360° corresponding to a full rotation of the component. This has the particular advantage that on reaching a position of 180° relative to the azimuth angle, the measuring signal can be doubled. If a full 360° rotation can be carried out, for reasons of symmetry a redundant series of measurements is obtained for the measurement at an angular range of 0° to 180°, which leads to a further statistical improvement in the measured values.
The component can be rotated in uniform azimuth angle steps, to simplify the evaluation of the series of measurements for the purpose of determining the inclination. For example, it is possible for measurements to be made in 10° steps relative to the azimuth angle, in order to obtain a series of measurements comprising 36 values.
The statistical elimination of measurement errors caused by vibrations of the tower is furthered when at any given azimuth angle a series of output signals is successively measured and recorded and then an averaged value is determined and recorded from the series of output signals and is used as a basis for the series of measurements. For example, in every given azimuth angle position in which a measurement is to be made, measuring can be carried out over three minutes at a high sampling rate. During averaging, time-dependent portions of the acceleration measured values for the most part then stand out.
In a development of the invention, in the method the evaluation can include the determination of a maximum value and of a minimum value from the series of measurements as well as the determination of the difference between the maximum and the minimum. In this respect, a maximum value of the acceleration signal will be expected when an acceleration sensor is aligned by the measurement axis thereof such that a maximum direction component is aligned vertically. The associated minimum value will conform with the corresponding measured value of the series for an azimuth angle shifted by 180°.
In particular, the inclination can be calculated according to the invention using the following formula:
  α  =      arcsin    ⁡          [                        1          2                ⁢                                            a              max                        -                          a              min                                g                    ]      α denoting the inclination angle, g denoting the gravitational acceleration, amax denoting the maximum value of the series of measurements and amin denoting the minimum value of the series of measurements. Geometric considerations, which are explained in detail further below in connection with the figures, show that the required inclination angle of the tower relative to the plumb line direction can be determined using this formula when values are evaluated at maximum and a minimum acceleration for a series of measurements which [ . . . ] for measuring points which belong to a full rotation of the rotatable component about an azimuth angle range of 0° to 360°. In this respect, the maximum value corresponds to an azimuth position of the rotatable component, in which the vertical component of the sensor measurement axis is aligned downwards parallel to the vertical. In contrast thereto, the minimum value corresponds to an azimuth angle value of the component, for which the vertical component of the sensor measurement axis is aligned vertically upwards, i.e. is directed counter to the force of gravity.
The method is substantially improved when the inclination is calculated using the following formula:
  α  =      arcsin    ⁡          [                        1          2                ⁢                                            a              max                        -                          a              min                                            g            ·                          cos              ⁡                              (                β                )                                                        ]      β denoting the tilting of the sensor measurement axis relative to the plane of rotation. Consideration of a possible tilting of the sensor measurement axis relative to the plane of rotation of the component is considered by means of this functional correlation, like geometric considerations which are explained in connection with the description of the figures. Thus, according to the invention, if the tilting angle β is known, the influence thereof can be exactly calculated in the evaluation of the series of measurements in order to calculate the inclination angle α.
In a preferred configuration of the method according to the invention, the tilting of the sensor measurement axis relative to the plane of rotation is determined from the series of measurements, preferably by determining the offset of a variable portion of the series of measurements, and will be considered in the determination of the inclination. Geometric considerations, which are explained in detail further below in connection with the figures, show that the result of tilting the sensor measurement axis relative to the plane of rotation of the component is that a part, varying with the azimuth angle in the case of the non-vanishing tilting β of the sensor measurement axis relative to the plane of rotation, is shifted in the form of an offset relative to the zero point. The determination of the offset from the series of measurements is quite possible with conventional signal processing using customary methods, well-known per se, for curve sketching.
In particular, according to the invention, the offset can be determined by forming the sum of the maximum value and of the minimum value and dividing the product by two. In the event of sinusoidal variation of the measuring signal, to be expected for geometric reasons during rotation, as a function of the azimuth angle, the portion, variable with the azimuth angle, with the vanishing tilting angle β is symmetrical about the horizontal axis, so that the sum of maximum value and minimum angle is zero.
In a preferred configuration of the method according to the invention, calculates for the tilting from the offset using the following formula:
  β  =      arcsin    ⁡          [                        a          0                g            ]      β denoting the tilting, g denoting the gravitational acceleration and a0 denoting the offset of the variable portion of the series of measurements. The offset can for geometric considerations regarding the measurement method according to the invention, which are described in further detail further below with reference to the figures, the offset value of the measured acceleration can be identified by the value which is obtained when the azimuth angle is adjusted such that the sensor measurement axis is aligned so that there would not be any inclination in the vertical direction of the sensor measurement axis for a vanishing tilting.
To increase the confidence interval of the measuring method according to the invention, a second acceleration sensor can be attached to the component such that the sensor measurement axis is oriented substantially parallel to the plane of rotation of the component and at a right angle to the sensor measurement axis of the first acceleration sensor, the output signal of the second acceleration sensor being measured and recorded in addition to the output signal of the first acceleration sensor, the inclination additionally being determined by a configuration of the second series of measurements obtained thus. Geometric considerations regarding the measurement method show that the series of measurements, which corresponds to the second acceleration sensor of the first series of measurements, shifted by 90° in respect of the azimuth angle. The evaluation of the series of measurements of the second acceleration sensor, which according to the invention is crossed relative to the first acceleration sensor, thereby provides a redundancy which can advantageously be used for the statistical averaging of the inclination values which have been obtained. Furthermore, many wind turbines are already provided with crossed acceleration sensors or integrated biaxial acceleration sensors. Therefore, the method according to the invention can advantageously be implemented in wind turbines of this type in that, during the rotation of the machine house, the output signals from these two acceleration sensors are to be evaluated. With regard to the considerations regarding the tilting of the second acceleration sensor relative to the plane of rotation, the analogous considerations and methods according to the invention for the determination thereof from the series of measurements apply, like the above-stated considerations on the first acceleration sensor.
A further improvement of the method according to the invention is obtained when a further acceleration sensor is attached to the component such that the sensor measurement axis is oriented substantially vertically to the plane of rotation of the component, the output signal of the further acceleration sensor being measured and recorded in addition to the output signal of the first acceleration sensor, the inclination additionally being determined [by] an evaluation of the further series of measurements obtained thus. According to the invention, this further acceleration sensor is aligned vertically to the first acceleration sensor and also vertically to a second acceleration sensor which may be used. The output signal from this further acceleration sensor may be theoretically invariant compared to a variation of the azimuth angle, out of mixed considerations, and therefore may be used as a reference for the series of measurements of the other acceleration sensors. In particular, the vertical acceleration sensor can be used to increase accuracy if there is a high level of cross-sensitivity of the sensors, in order to reduce the influence of the cross-sensitivity. ‘Cross-sensitivity’ is understood as meaning that an acceleration sensor is also sensitive to accelerations which occur at a right angle to the sensor measurement axis.
Finally, in an advantageous configuration of the method according to the invention, the azimuth angle can be determined for the output signal and the series of measurements, the second series of measurements and/or the further series of measurements can be supplemented by recording the azimuth angle associated with each output signal, to produce a function graph consisting of ordered pairs, to calculate a direction of the inclination. Whereas it is thus not necessary for the basic principle of the method according to the invention to record absolute values of the azimuth angle, since only maximum and minimum values as well as offsets are used for the evaluation, a determination of the azimuth angle can be used absolutely for characterising the alignment of the sensor measurement axis. Inter alia, this allows an inclination direction to be established.
Finally, the object of this invention is achieved by a device for implementing the method of the invention according to any one of claims 1 to 12. The device comprises an evaluation unit as well as means for reading out output signals from the acceleration sensors which are either already components of the wind turbine to be measured or which, as components of the device according to the invention, are to be connected to the wind turbine to be measured. The evaluation unit is configured to carry out the evaluations described in the method claims.
The invention is described by way of example in a preferred embodiment with reference to drawings, and further advantageous details can be seen in the figures of the drawings.
Functionally identical parts have been provided with the same reference numerals.